Inverted hexagonal, lipid structure

FIG. 7 Structures of various liquid-crystalline phases of membrane lipids. (A) Normal hexagonal phase (Hi) (B) lamellar phase (C) inverted hexagonal phase (Hu). Cubic phases consisting of (D) spherical, (E) rod-shaped, and (F) lamellar units. The hydrocarbon regions are shaded and the hydrophilic regions are white. (Reprinted by permission from Ref. 11, copyright 1984, Kluwer Academic Publishers.)... 

Cyclic carbohydrates with two alkyl chains (e.g. 1,2-dialkyl (or 1,2-diacyl) glycerol 8 a (sug=Glcp, Galp) present structural similarities with glycerophospho-lipids. They form complex mesophases such as bicontinuous cubic phases, inverted hexagonal phases or myelin figures [58-61]. Other dialkyl derivatives... 

Seddon, J. M. (1990). Structure of the inverted hexagonal (HII) phase, and non-lamellar phase transitions of lipids, Biochim. Biophys. Acta, 1031, 1-69. 

CL-DNA complexes form spontaneously when solutions of cationic liposomes (typically containing both a cationic lipid and a neutral helper lipid) are combined. We have discovered several distinct nanoscale structures of CL-DNA complexes by synchrotron X-ray diffraction, three of which are schematically shown in Fig. 1. These are the prevalent lamellar phase with DNA sandwiched between cationic membranes (Lo,c) [22], the inverted hexagonal phase with DNA encapsulated within inverse lipid tubes (Hnc) [23], and the more recently discovered Hj0 phase with hexagonally arranged rod-like micelles surrounded by DNA chains forming a continuous substructure with honeycomb symmetry [24]. Both the neutral lipid and the cationic lipid can drive the formation of specific structures of CL-DNA complexes. The inverse cone shape of DOPE favors formation of the... 

The internal structure of the complexes can directly determine the mechanism of transfection [4, 23, 25]. We have found that for Lac CL-DNA complexes, the membrane charge density (aM) is a predictive parameter for transfection efficiency [21] (see Sect. 2), i.e., the data for monovalent and multivalent cationic lipids are described by a simple bell-curve. In contrast, for inverted hexagonal HnC CL-DNA complexes, TE is independent of aM, suggesting a distinctly different mechanism of transfection. Consistent with the TE data, confocal microscopy revealed distinctly different CL-DNA complex pathways and interactions with cells, which depended on both the structure (HnC vs Lac) and, for Lr/ complexes, on aM [25]. Thus, the mechanism of transfection by CL-DNA complexes is dependent both on their structure and, for a given structure, on chemical and physical parameters of the complexes. 

Figure 4.2 Lipid phases, (a) Gel (Lp) lamellar phase, (b) Liquid-crystalline Lj) or fluid lamellar phase, (c) Inverted hexagonal (Hn) phase, (d) Hexagonal phase. [Reproduced from R.B. Gennis (1989) Biomernbranes Molecular Structure and Function. Springer New York, (p. 41), with permission]...

The lamellar phase represents the structure of cell membrane lipids under steady-state conditions. However in certain circumstances, particularly in membrane fusion events (e.g. in egg fertilization, or cell infection by some viruses), membrane lipids abandon transiently the lamellar disposition, adopting nonlamellar architectures, of which the best known is the so-called inverted hexagonal , or Hn, phase. Nonlamellar structures are at the origin of the lipid stalk , a structural intermediate that connects two bUayers in the membrane fusion process. Only certain lipids, or lipid mixtures, can undergo the Lo(-Hii thermotropic transition, and the latter can be detected by DSC. Hu, like other nonlamellar phases, has received particular attention lately because of its possible implication in important phenomena such as cell membrane fusion, or protein insertion into membranes. High-sensitivity DSC instruments allow the detection of La-Hn transitions with phospholipid suspensions of concentration 5 him or even less. 

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